Imaging paper, particularly photographic imaging paper, requires materials in the image substrate that provide long-term survivability and stability during both display and storage. These properties are most desirable and have significant commercial value.
It has been proposed in U.S. Pat. No. 5,244,861 to utilize biaxially oriented polypropylene sheets laminated to cellulose photographic paper for use as a reflective receiver for the thermal dye transfer imaging process. In the formation of biaxially oriented sheets described in U.S. Pat. No. 5,244,861, layers of polypropylene are cast against a water-cooled roller and quenched by either immersion in a water bath or by cooling the melt by circulating chilled liquid internal to the chill roll. The cast polymer sheet is then stretched in the machine direction and then stretched in the cross direction. The sheet is then annealed and is wound in roll form ready to be laminated to a paper base substrate. One material that offers excellent orientation properties is polypropylene. While polypropylene offers excellent physical properties, one disadvantage is its thermal and light stability, particularly when TiO.sub.2 is present in one or more of the layers. Polypropylene is usually stabilized using a phenolic based antioxidant, but this material does not offer sufficient stability for light keeping and dark keeping yellow edge. In the area of coextrusion of multiple layers, there is a significant advantage over a mono layer of polymer in that the layer(s) containing TiO.sub.2 can be reduced in thickness, and the concentration of TiO.sub.2 can be significantly increased to achieve the desired sharpness and calorimetric properties. The coextrusion process allows for an overall reduction in expensive pigment materials while achieving superior results. When less material is used, the degree of objectionable yellow edge and overall stability is also reduced. Furthermore, in a coextruded structure, a clear layer of polymer can be placed directly over the layer containing pigment and antioxidants. Sealing the surface of polymer from the atmosphere can also help to significantly reduce the amount of dark keeping yellowing.
While a variety of materials may be used to create a coextruded structure, one of the preferred materials is polyethylene because of its chemical inertness during photo processing. Coextrusion further allows the use of dissimilar materials to be extruded simultaneously in a single pass on a machine. Layers such as polyethylene with polyester and/or with polypropylene may add desired strength and optical properties, handling features such as durability, and resistance to long-term degradation. Furthermore, the use of dissimilar materials may provide desired barriers to the transmission of gases such as oxygen, water vapor, carbon dioxide, nitrogen, and other compounds that can create interactions with various chemistries in or on the polymer layers or in the image layer. In addition, the low cost of polyethylene material makes it attractive to use, but only if it is properly stabilized to provide resistance to thermal degradation for polymer processing during manufacture, but also to provide a stable base for light stability, as well as dark keeping.
In U.S. application Ser. No. 08/862,708 filed May 23, 1997, now U.S. Pat. No. 5,866,282 it has been proposed to use biaxially oriented polyolefin sheets laminated to photographic grade paper as a photographic support for silver halide imaging systems. In U.S. application Ser. No. 08/862,708 filed May 23, 1997, advantages including increased opacity, improved tear resistance, and reduced substrate curl are obtained by the use of high-strength biaxially oriented polyolefin sheets. The optical advantages of biaxially oriented polyolefin layers are realized when an opacifying pigment is located in at least one layer of polymer, which may be solid or voided. Either the rutile or anatase crystalline form of titanium dioxide (TiO.sub.2) is commonly used for opacity, whiteness, image sharpness, and control of pearlescence. While all these are possible, the coextrusion or series extrusion of a plurality of layers directly onto a substrate provides a simple one-pass process in which the polymers are converted from pellet form to usable layers that are cast onto a substrate without orientation. Since most biaxially oriented machines have a relatively fixed width and, therefore, degree of orientation, the casting of layers directly on paper provides added latitude in the materials that can be used since they are not limited to their ability to be oriented.
Coextrusion is a process wherein more than one melt extruder or pump are used to melt polymers and then the individual melt streams are jointed in a feed block prior to the inlet of an extrusion die. The layers are then cast simultaneously onto a web substrate in a roller nip. Usually there is a temperature controlled roller in the nip that applies pressure to aid in the solidification of the melt polymer layers. In series extrusion usually one layer at a time is applied to the web by melting the polymer and casting it onto the substrate. A series of extruders are used to achieve multilayers on the web.
All polymers are inherently prone to chemical degradation that leads to loss of mechanical properties. They undergo thermal degradation during processing such as extrusion of thin films and photooxidative degradation with long-term exposure to light. TiO.sub.2 catalyzes and accelerates both thermal and photooxidative degradation. In the art of resin coating a single layer or coextrusion of multiple layers of polymers onto photographic paper, the melt polymers are extruded at high temperatures and are subjected to high shear forces. These conditions may degrade the polymer, resulting in discoloration and charring, formation of polymer slugs or "gels", and formation of lines and streaks in the extruded film from degraded material deposits on die surfaces. Also, thermally degraded polymer is less robust than nondegraded polymer for long-term stability, and may thereby shorten the life of the print.
Hindered phenol antioxidants are commonly used alone or in combination with secondary antioxidants to stabilize polymers during melt processing, but provide little protection from long-term photooxidation. They are also responsible for some forms of oxidative atmospheric gas yellowing (dark keep yellowing) in prints stored in the dark. This undesirable color may develop on the print or around the print edge with archival keeping, and has been attributed to colored oxidation products of phenolic antioxidants that are formed in the dark in the presence of white pigments such as TiO.sub.2.
In U.S. Pat. No. 4,582,785 it is suggested that polymeric hindered amines as the sole stabilizer, when added to polyethylene coated photographic paper, can improve their photostability. In this patent a polymeric hindered amine is claimed as the sole stabilizer for both thermal processing and light stability in a single layer of a polymeric material, preferably polyethylene. Photostabilizers such as the polymeric hindered amine improve the archival qualities of the resin layer, but because of their high cost have not been economical in a single thick pigmented layer of polymer, thereby severely limiting their use. Another disadvantage is that with a mono layer of polyethylene, excessive quantities of TiO.sub.2 and HALS are required making the material very expense. In addition, these levels can also interfere with the adhesion of the polymer layer to the base substrate or the emulsion to the polymer layer.
There remains a need to provide an imaging support that contains a plurality of polymer layers, some of what may contain pigments and/or voids and that are extrusion processable with minimal degradation of polymer. In addition, the polymer layers must have exceptional long-term resistance to degradation and embrittlement when exposed to light and other environmental stresses, while providing an imaging support that has exceptional dark stability and prevents discoloration during dark keeping.